1. Field of the Invention
The present invention relates to an NMR detector used in an NMR spectrometer and to an NMR spectrometer equipped with the NMR detector.
2. Description of Related Art
An NMR spectrometer is an apparatus for gaining an NMR spectrum by placing a sample under investigation within a static magnetic field, irradiating the sample with an RF pulse, and then detecting a feeble RF signal (NMR signal) emanating from the sample. The molecular structure can be analyzed by extracting molecular structural information contained in the spectrum.
FIG. 1 schematically shows the structure of an NMR spectrometer having an RF oscillator 1 emitting an RF pulse. The phase and amplitude of the RF pulse are controlled by a phase controller 2 and an amplitude controller 3 and sent to a power amplifier 4.
The RF pulse is amplified to a power necessary to excite an NMR signal by the power amplifier 4 and then sent to an NMR probe 6 via a duplexer 5. The pulse is directed at a sample under investigation (not shown) placed within the NMR probe 6. After the RF irradiation, a feeble NMR signal emanating from the sample is sent to a preamplifier 7 again via the duplexer 5 and amplified up to a signal intensity permitting reception.
A receiver 8 converts the frequency of the RF NMR signal amplified by the preamplifier 7 into an audio frequency that can be converted into a digital signal. The audio frequency of the NMR signal converted by the receiver 8 is converted into a digital signal by an analog-to-digital (A/D) converter 9 and sent to a control computer 10.
The computer 10 controls the phase controller 2 and amplitude controller 3 and Fourier-transforms the NMR signal accepted in the time domain. The computer automatically corrects the phase of the Fourier-transformed NMR signal. Then, the signal is displayed as an NMR spectrum.
FIG. 2 shows the structure of the prior art NMR detector. The NMR probe has the detector 11 in which a detection coil 13 is mounted to emit an RF pulse for exciting a sample 12 to be investigated and to detect an NMR signal emanating from the sample 12. The detection coil 13 cooperates with a tuning and matching circuit 14 to constitute an RF resonant circuit. This RF resonant circuit sends and receives RF pulses and NMR signals to and from the sample 12 contained in an NMR cell 15 placed within the probe 11. Where the sample contained in the cylindrical NMR cell is in the form of a solution, high sensitivity can be obtained by this method of measurement. However, there is the problem that where the sample assumes a planar form, high sensitivity cannot be obtained.
In recent years, an NMR detector using a meander coil has been proposed to detect an NMR signal originated from a planar sample at high sensitivity (see U.S. Pat. No. 6,326,787). FIGS. 3A–3C show one example of an NMR detector using a meander coil. FIG. 3A is a plan view of the detector; FIG. 3B is a cross-sectional view taken in a lateral direction as indicated by the broken line in FIG. 3A; and FIG. 3C shows the direction of RF magnetic field around coil wires. In FIGS. 3A–3C, a base plate or substrate 16 forms a sample cell. The base plate 16 is made of an insulator such as a glass plate of low dielectric loss to achieve high-sensitivity NMR measurements.
A meander coil 17 consisting of an elongated conductor repeatedly bent into comb teeth-like straight segments which are regularly spaced from each other and which are uniform in length is mounted on the surface of the base plate 16. The segments at both ends of the meander coil 17 extend downward and are placed opposite to each other under the bent portions. A capacitor 18 made of a dielectric is bridged across the opposite ends. An LC resonant circuit is formed by the inductance L of the coil 17 and the capacitance C of the capacitor 18. Thus, if radio waves are injected into the meander coil 17, an RF magnetic field B1 is produced across the meander coil as indicated by the arrow in FIG. 3C. An RF magnetic field B1 is applied to a planar sample 19 placed on the coil 17. If a static magnetic field B0 is previously applied parallel to the plane of the paper, an NMR signal from the sample 19 can be observed by interaction with the RF magnetic field B1.
In the prior art, the sample space extends planarity. In the sample space, in directions crossing the meander coil, the phase of the produced RF magnetic field is rotated. Therefore, there is the problem that if the sample diffuses in a direction crossing the meander coil, the intensity of the NMR signal decreases. Especially, in a case where the spacing between the adjacent segments of the bent conductor, or meander coil, is small, this effect is conspicuous. Therefore, samples making use of the feature of the meander coil, i.e., high sensitivity to trace amounts of sample, have been limited to solid samples.